1. Field of the Invention
Embodiments of the present invention relate to a method of testing a transformer prior to installation in a high-pressure environment and a transformer.
2. Description of the Prior Art
In underwater, for example subsea, electrical power distribution applications, transformers are increasingly used in pressure-compensated enclosures. The transformer is housed in an enclosure containing oil, and when deployed under water, the oil pressure is made equal to the external water pressure so the transformer may therefore operate in oil at very high pressures, for example equivalent to 3,000 m depth or more. The magnetic core of the transformer is typically formed from varnish-covered core-elements, and such high pressures can have a damaging effect upon these. Such varnished-covered core-elements are typically shaped as “I” and “E” profiles, though other form-factors may be used. The core elements may be formed from metals such as steel, or nickel/iron alloys etc.
FIGS. 1 to 3 illustrate a typical simple 50 Hz transformer construction with an iron/nickel alloy core. This comprises a plurality of laminations, typically between 0.5 and 0.35 mm thick. The laminations shown comprise core-elements of the so-called the “I” and “E” profiles, 1 and 2 respectively. During the assembly process shown schematically in FIG. 2, for each lamination, the centre arm 3 of the “E” core-element 2 is passed through the centre of dual bobbins 4 and 5, which carry the required windings. The “E” core-element 2 is arranged to butt up to the “I” core-element 1. Each lamination is assembled in the reverse sense to its adjacent lamination(s), as shown in FIG. 2, where for the second layer of laminations, the “E” core-element 6 is assembled in the opposite direction to the first “E” core-element 2 and butts up to an “I” core-element 7 at the opposite side of the bobbins 4, 5 to the first “I” core-element 1. The process is continued to form a stack of laminations, and the complete assembled stack is held together with nuts 8 and screwed rods 9 (shown in FIG. 3) located through holes 10 in the core-elements. An end-on view of the transformer when partially assembled is shown in FIG. 3.
One of the most common pressure-related failure modes is as follows: under pressure, the core-elements may be “pushed” one against the other, such that there is a possibility of the varnish being damaged. This can result in short-circuits between the core-elements and, consequently, higher than normal induced electrical currents, which may cause the core to heat up. This temperature increase may dramatically decrease the efficiency of the transformer and could result in its destruction.
One known solution to this problem is to use pressure-testing facilities prior to installation of the transformer. Here, a transformer is placed in a pressurised housing, the pressure being chosen to best simulate the ambient pressure of the installation environment. However, these facilities are very expensive to use and hire, and indeed many transformer manufacturers do not have such a facility.
Embodiments of the present invention provide a technique to reduce transformer failures in relatively high ambient pressure environments. This aim is achieved by testing transformers to identify potential failures prior to deployment, by simulating the high barometric pressure that the core elements will be subjected to when the transformer is installed, for example at a subsea location. Unlike known pressure-testing facilities, embodiments of the present invention make use of a mechanical compression force applied to the transformer.
This simulation is achieved by the temporary application of a compression force on the laminations of a transformer. This may be achieved for example by tightening lamination securing hardware and spreading the compression force across the laminations to a point where the compression force is at least similar to that which the transformer will be subjected to by ambient pressure at installation. Thus the applied compression simulates the conditions that the laminations are subjected to when the transformer is installed subsea. The transformer is tested electrically, for example during or after the applied lamination compression, to reveal any increase in losses which have resulted from any short circuits between laminations which have been caused by the high compression.